1. Field of the Invention
The invention relates to the field of telephone systems and in particular to telephone systems in which multiple stations may be operated on a single shared telephone line.
2. Description of the Prior Art
The System
Home and small office telephone systems are typically purchased and operated by the telephone user. In addition, the telephone user or the owner of the building in which the telephone system is installed, is similarly responsible for the installation and maintenance of the internal telephone lines beginning at the telephone company line terminal at the entry point to the building. Very commonly, the internal telephone wiring which is available in a home or small office is a single line system. In other words, while there may be multiple telephone jacks, each of the jacks are coupled in parallel to a single telephone line within the structure. Therefore, only one telephone conversation at a time may be carried on the line and if two handsets are picked up, the power on the line is distributed between the two activated handsets with a consequent loss of audible volume. Additionally, the communication as well as the central office link must be generally held in common between the two participants.
A number of schemes have been attempted for expanding the communication capability of a single internal telephone line network, most of which involved various voice band multiplexing schemes. As the features and complexity of the telephone network increases, greater demand is placed upon the multiplexing system. The complexity of the multiplexing scheme can become prohibitive when the system must be full featured. For example, when the system must provide an automated operator to provide voice message answering of incoming calls, special handling of fax, phone, modem and answering machines at the telephone stations, extension dialing, a host of separate business phone features and fax features, and accommodate a large number of phone stations all incorporated in an economical package which easily lends itself to expansion and modification, the prior art has failed to provide practical solutions. The difficulty arises from many reasons including the following: (1) The expense and labor involved in wiring or rewiring the facility if a star wire system is used (a separate wire to each phone); and (2) the complexity and technical difficulty in implementing a digital or frequency band (AM, FM, etc.) form of sharing a single common line. Finally, the high communication and computer overhead time which is required in a multiplexing system to handle multiple features and stations with complete flexibility and adaptability. The amount of digital communication which must be carried on often becomes prohibitive and the system fails during peak periods or unusual demand scenarios.
Many multiple wire (star wire) as well as some two-wire small phone systems utilizing a master controller and multiple station controllers are known in the art. The multitude of star wire systems suffer from both the inherent wiring complexity as well as the following problem. One typical two wire system is the Model 8002 MCD base key telephone system unsuccessfully offered by Rockwell. In such systems, a sophisticated processor is not only required in the master controller, but also at each station which adds to the cost and complexity of the system. An architecture which is built upon multiprocessing makes changes in the system difficult since modifications must be made in both the controller program and in the station programs. Communication between these multiprocessors is complex and when the number of stations increases, overall communication can become very slow during busy periods. Only a few features can be changed remotely since again most of the features require changes in both station and controller programs.
In the Rockwell system, a mix of voice channels, digital channels and a reference signal on a twisted pair telephone line, using double side band suppressed carrier with amplitude modulated signals was employed. However, the Rockwell system did not define the structure of digital data communication, nor did it remove the requirement for a separate control processor in every station. Therefore, although it did allow multiple signals on a single twisted pair telephone line, it did not provide any simplification to the complexities of multiprocessing nor solve the inflexibilities inherent in multiprocessing architectures. Additionally, this product suffered from severe noise, phase lock, and synchronization problems which eventually doomed it to failure.
Therefore, what is needed is a single twisted pair, multichannel telephone system which can be economically and easily adapted to a home or small business and which has the power and flexibility to perform the functions discussed above without undue complexity, expense or susceptibility to failure under heavy demand or unusual use scenarios. Additionally, it needs to solve the critical noise and synchronization problems inherent in such a system without resort to a full digital (and very expensive) solution.
Simplified Processor
Conventional general purpose processors typically operate on a program stored in a read only memory by means of an instruction counter in order to read the stored instructions in sequence or according to a sequence with programmed jumps. This architecture is relatively complex, requires large numbers of transistors to implement and will therefore use a large area of a chip when integrating this function, and is time consuming of real time communication cycle time. The disadvantages of such a processor are particularly burdensome in an application where general programmability is not required.
What is needed is an architecture and method of operation for a processor which is more usable for applications that require only a limited number of operations and that avoids the overhead and timing disadvantages of a general purpose processor.
Communication Signaling Scheme
In a typical prior art small telephone system, a separate pair of wires is coupled from a control unit or master phone to each remote station or telephone handset. This type of system is commonly referred to as a "star wire" system. The control unit determines where the message is coming from and where it will be going according to which pair of wires is selected. This system has the disadvantage that there is a need to provide a separate wire pair for each remote station connected to the control unit and thus eliminates the use of standard house telephone wiring since conventional home wiring typically connects to all the extensions or telephone stations in parallel on a single pair of wires.
The prior art has also devised a scheme in which a single pair of wires is shared between multiple remote stations connected to a control unit. The control unit or master phone manages the telephone network by using a message based protocol. Whenever a phone call or message is to be sent, the transmitting unit, regardless of whether it is a remote station or control unit, will initiate a message in the network by sending an initial data protocol which will define the transmitter and recipient of the message. This type of message based protocol is subject to slow downs or lock up as the number of remote units and systems activity increases.
What is needed is a communication protocol for a small telephone system which is not subject to the limitations of the prior art. In particular, the protocol should eliminate the need in the system to establish a handshake protocol every time a message is sent in either direction to avoid slow downs during active communication periods between a plurality of units. Such a protocol should also allow serial digital data to be transmitted over the same line in burst format without affecting signaling speed so that components such as a display can be serviced quickly without affecting signaling response time. Additionally, a good error correction scheme must be implemented-without affecting signal response time. Serial data should be quickly transferred for display or use in other serial communication network applications regardless of the direction of transfer.
Voltage Controlled Crystal Oscillator
Voltage controlled crystal oscillators in the prior art generally do not use MOSFETs to modulate the output impedance of the amplifier stages in the oscillator to shift the phase of the output in order to change the frequency of the crystal controlled oscillator. In addition, prior art voltage control oscillators are generally not designed to be easily integrated in large communication circuits.
What is needed is a crystal controlled voltage oscillator using phase shift techniques in which the frequency of oscillation can be adjusted within a tight and stable range and which has a topology suited to integrated circuitry and that utilizes a small chip area.
Light Emitting Diode Driver Circuit
The standard method for driving multiple light emitting diodes (LEDs) is to drive them in parallel through a series resistance and switched by appropriate logic signals. The current through any light emitting diode is determined by the value of its corresponding series resistor when the logic switch is closed which is also in series with the diode. When all the light emitting diodes are on, the current is additive and may be substantial depending upon the number of LEDs.
Therefore, what is needed is a circuit in which the total current used to drive a bank of light emitting diodes can be held constant and limited for lower power applications. Further, if the mechanism using the diodes is line powered on a line also used for communication the amount of noise which switching of the current through the light emitting diodes places upon that communication line should be avoided as much as possible as it could interfere with that communication. In addition, the current supplied to each LED must be maintained as constant as possible in order to maintain consistent LED brightness within a multiple bank of LEDs regardless of how may of the LEDs may be lit. These attributes are difficult to maintain with parallel LED switching.
Automatic Timing Compensation for a Communication Line
Telephone systems which use an internal telephone cable and which carry voice and/or data modulated signals at high frequencies are characterized by line delays between signals transmitted between the control unit and the station unit and between two station units. In order to compensate for these inherent line delays, the prior art has devised circuits for advancing the transmission signal or delaying the received signal a fixed average amount given the line length specification variations if the line delay is above a predetermined minimum and otherwise accepting smaller line delay inaccuracies.
Another method used in the prior art is to manually adjust the compensation components tied to the line at the time of installation in order to attempt to cancel out line delay variations.
What is needed is some type of circuit which provides for automatic adjustment to the transmission signal to avoid line delays, a circuit which can be implemented at low cost and which adjusts for potential load changes such as may occur when a new station is plugged in.
A Low Cost Adaptive Echo Balance Methodology
Whenever there are four-to-two wire conversions in a telephone system, there is an echo back of the transmitted signal which must be given consideration in the design of the system. In such applications where echo in not critical, the provision of a fixed component balance network is usually a satisfactory solution. In other applications where echo cancellation is more critical, normally a signal processor particularly adapted to cancel the echo is employed.
What is needed is some type of apparatus and methodology wherein echo cancellation can be achieved in applications where a fixed network would not provide a satisfactory solution, but which does not require the more expensive compensation based upon a digital signal processor for echo cancellation.
A Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled, Communication Line Interface
A typical prior art means for interfacing multiple signals onto an internal (inside the house or office) twisted pair communication line is comprised of a transformer across the primary of which the internal communication line is connected and across the secondary of which a transmit buffer and receive buffer are provided for bidirectional communication with internal circuitry. The internal communication line could be a telephone cable within a home or office and the internal circuitry to which it would be coupled would be a key telephone circuit or handset. A plurality of such transmit and receive circuits could be coupled in parallel across the secondary of the transformer to couple multiple sources at the same location to the internal communication line through a single interface. Each source to be coupled to the internal communication line at different locations of course requires a separate transformer and in a typical system, a multiplicity of transformers are used corresponding to the number of remote sources.
The large number of transformers necessary for a useful system creates a large accumulation of magnetizing inductance, leakage inductance, core saturation and resonance which degrades the communication transmission for relatively wide bandwidth signals and thereby severely limits the number of interfaces that can be reliably coupled to the internal communication line.
Therefore, what is needed is a circuit and method to provide interfacing for a two-wire twisted pair that will support: (1) multiple signals being communicated on the line at the same time through the same interface such as a reference frequency, digital signaling data, and/or voice modulation at different frequencies; (2) signals being communicated through several interfaces simultaneously from different locations on the line; (3) large variation in line load and other system loads without significant signal degradation; (4) the supplying DC power over the same lines; (5) a common low impedance line load; and (6) echo cancellation to eliminate false data from being transferred.
Telephone Line Interface
A typical telephone line interface uses a transformer to couple to the central office lines and two buffer amplifiers coupled to the secondary of the transformer to provide a signal OUT and signal IN to the equipment being interfaced whether a common telephone or a sophisticated PBX system. An example of such a prior art line interface unit is shown in FIG. 21 and includes transformer 338 with a secondary 340 and primary 342. Amplifier 344 is used to generate the signal OUT and is coupled to secondary 340 while amplifier 346 drives the secondary from the signal IN. Summing resistors 348 associated with the transmit amplifier 344 sums the input signal and the output signal to thereby provide basic echo cancellation. Output drive amplifier 346 and its associated summing resistors 350 and output resistor 352 drive the appropriate output signal through the transformer 338 onto the telephone lines coupled to primary 342.
The implementation of FIG. 21 suffers from an erroneous echo signal due to reactive parameters associated with the greatly varying telephone line characteristics and transformer 338.
A Piezo Driver Using Voltage Doubling and CMOS Techniques
A typical prior art piezo circuit uses switched positive and/or negative supply voltages which are available within the system, or alternatively some type of power supply which is coupled to the piezo driver to provide a higher voltage which is switched in when needed. This is a relatively expensive and space consuming solution to the problem of providing a higher voltage to a piezo ringer than is normally available in a telephone circuit.
Therefore, what is needed is some means for applying a higher voltage to the piezo element without the need of providing an additional higher voltage power supply so that sound volume improvement can be provided in a manner compatible with integrated circuit technology at low cost.
Voltage Limiter
Prior art voltage limiters for amplifiers typically use cascaded diodes coupled in parallel across the input and output of the amplifier in one or both directions to limit the voltage range of the amplifier. The linear range of such cascaded diodes is, however, limited.
Therefore, what is needed is some type of circuitry in which voltage limitation across an amplifier can be achieved over an extended linear dynamic range, closer to the voltage limit points.
Line Powering for Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled Communication Line Interface
When remote electronic devices are powered by the communication line, consideration must be given to the noise generated by providing power on the same wires as the communication signals. In a typical house telephone line, this type of noise is typically not severe since all the operating devices share the same communication channel and the same parties to the communication. In that case, any noise is synchronous with a voice conversation in progress and is generally not harmful. In those cases where the line is shared between devices having signals which are incompatible, such as in a telephone and modem communication, the user generally does not use the incompatible devices at the same time and thereby avoids the conflict. However, in the case of simultaneous, unrelated communication of multiple devices sharing the same wire, line powering becomes very complicated and the solutions are costly if low noise is required.
In a two-wire line, one wire is provided at a positive voltage and the other at negative voltage to provide a constant current flow to the remote stations. In order to energize such a system, the voltage must first be placed across the two wires of the line by a line power source. Then the remote stations must have means for pulling power from the line at a constant predictable rate in order to minimize the possibility of generating noise on the line which might interfere with the communication.
Digital and analog circuits typically utilize a constant voltage and have variable current demands. In addition, many such circuits require several different voltages to be supplied each with different current demands at different times. It is further desirable to minimize the different types of voltage supplies needed and to provide a current balance between the positive and negative rails as much as possible in order to avoid drift of the voltage supplies relative to the voltages of the supplying two-wire lines.
Therefore, what is needed is some sort of apparatus and methodology for line powering whereby multiple devices can employ a common line with multiple channels having unrelated signals, which devices communicate with different parties without the generation of noise through the line powering which interferes with any of the communication.